如何使用C＃3.5框架更新App.config？

<?xml version=\"1.0\" encoding=\"utf-8\" ?>
<paths>
  <path name=\"pathtofile1\">
    <fullpath>\\\\machine1\\folder1\\file.txt</fullpath>
  </path>
  <path name=\"pathtofile2\">
    <fullpath>\\\\machine2\\folder2\\file2.txt</fullpath>
  </path>
</paths>

XDocument doc = XDocument.Load(pathToXmlfile);

var filePath1 = from c in doc.Descendants(\"path\")
                        where (string)c.Attribute(\"name\") == \"pathtofile1\"
                        select (string)c.Element(\"fullpath\").Value;

                string thePath = filePath1.First();

var appConfigPath = string.Format(\"{0}{1}.exe.config\", Directory.GetCurrent(), Process.GetCurrentProcess().ProcessName);

var appConfig = XDocument.Parse(appConfigPath);

//have at it

System.Configuration.Configuration config = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration(\"~\");

if (config.AppSettings.Settings[\"YourTag\"] == null)
            {
                config.AppSettings.Settings.Add(\"YourTag\", \"yourValue\");
            }
else
            {
                config.AppSettings.Settings[\"YourTag\"].Value = \"yourValue\";
            }